1. Field of the Invention
This invention relates generally to devices that emit electromagnetic radiation and, in particular, to optically coupling one or more surface-emitting lasers to corresponding optical receiving devices such as amplifiers or modulators.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There arc several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. The possibility of amplification of electromagnetic waves in a semiconductor superlattice structure, i.e., the possibility of semiconductor diode lasers, was predicted in a seminal paper by R. F. Kazarinov, et al., xe2x80x9cPossibility of the Amplification of Electromagnetic Waves in a Semiconductor with a Superlattice,xe2x80x9d Soviet Physics Semiconductors, vol. 5, No. 4, pp. 707-709 (October 1971). Semiconductor laser technology has continued to develop since this discovery.
There are a variety of types of semiconductor lasers. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers of various types may be electrically pumped (by a DC or AC current), or pumped in other ways, such as by optically pumping (OP) or electron beam pumping. Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide.
Additionally, semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation is output perpendicular to the wafer surface.
One type of SEL is the vertical cavity surface emitting laser (VCSEL). The VCSEL structure usually consists of an active (gain) region sandwiched between two distributed Bragg reflector (DBR) mirrors. The DBR mirrors of a typical VCSEL can be constructed from dielectric or semiconductor layers (or a combination of both, including metal mirror sections). Other types of VCSELs sandwich the active region between metal mirrors. The space between the reflective planes is often referred to as the resonator. VCSELs typically have a circular laser beam and a smaller divergence angle, and are therefore more attractive than edge-emitting lasers in some applications. Further background discussion of VCSELs and related matters are found in: U.S. Pat. No. 5,468,656 (1994), Shieh et al., xe2x80x9cMethod of making a VCSELxe2x80x9d; U.S. Pat. No. 5,985,686 (1999), Jayaraman, xe2x80x9cProcess for manufacturing vertical cavity surface emitting lasers using patterned wafer fusion and the device manufactured by the processxe2x80x9d; MacDougal et al., xe2x80x9cElectrically-Pumped Vertical-Cavity Lasers with AlOxe2x80x94GaAs Reflectors,xe2x80x9d IEEE Photonics Letters, vol. 8, No. 3, March 1996. A variant on the standard VCSEL is the vertical-external-cavity surface-emitting laser (VECSEL). VECSELs are described in J. Sandusky and S. Brueck, xe2x80x9cA CW External-Cavity Surface-emitting Laser,xe2x80x9d IEEE Photon. Techn. Lett. 8, 313-315 (1996).
Semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. Electrons in the active region attain high energy states as a result of the potential applied. When the electrons spontaneously drop in energy state, photons are produced. Some of those photons travel in a direction perpendicular to the reflective planes of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times. When those photons interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If most electrons encountered by the photons are in the high energy state, the number of photons traveling between the reflective planes tends to increase. A typical laser includes a small difference in reflectivity between its mirrors, giving rise to amplification of light and thus lasing. The primary laser output is emitted through the reflective plane having lower reflectivity.
The use of semiconductor lasers (both edge-emitting and surface-emitting) for forming a source of optical energy is attractive for a number of reasons. For example, diode lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, the diode laser is a monolithic device, and does not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
VCSELs are used in a variety of applications. In telecommunications, for example, output laser light of a precise wavelength is modulated to encode and transmit information. The laser may be externally modulated, or directly modulated. For external modulation, an optical modulator receives and then modulates the output of the laser. The signal may also be amplified, e.g., by an SOA (semiconductor optical amplifier), Raman amplifier, or Erbium-doped fiber amplifier. The modulator and amplifier may be used in series (in either order), or independently.
A typical telecommunications system uses optical fiber to guide the radiation from the modulation (or emission) point to the detection point. In any event, it is desirable to couple the output of the laser into a signal-conditioning, signal-receiving optical devices, such as an SOA or modulator. In cases of an array of lasers, e.g. a VCSEL array, it is desirable to couple the output of each laser to a corresponding optical receiving device.
There is, therefore, a need for improved methods and systems for coupling the output of each laser, e.g. of a VCSEL array, to an optical receiving device.
The present invention is directed to a method and system for conditioning the output signals of an array of surface-emitting lasers with an array of corresponding edge-receiving optical devices. Both the array of surface-emitting lasers and the array of edge-receiving optical devices are mounted on an optical bench substrate, for alignment and coupling. The array of edge-receiving optical devices may also be monolithically fabricated on the optical bench substrate. The array of surface-emitting lasers and the array of edge-receiving optical devices are aligned by alignment features, which are fabricated on the optical bench substrate so as to align and optically couple the array of surface-emitting lasers to the array of edge-receiving optical devices.
An advantage of the invention is the ability to separately optimize the performance of the laser and modulator.
Another advantage of the invention is that it enables the output of a surface-emitting laser to be conveniently modulated.
Another advantage of the invention is that it permits coupling a high percentage of power from a surface emitting laser to an external modulator or other device.
Another advantage of the present invention is allowing data modulation rates higher than direct modulation limits.
Another advantage is aligning arrays of surface emitting lasers with arrays of modulators and, in particular, aligning an array of surface-emitting lasers with a corresponding array of edge-receiving optical devices.
Still another advantage is decreasing the cost of manufacturing lasers with high modulation rate capacity.
Another advantage is that it allows coupling to an SOA which amplifies the output of the modulator.
Other and further features and advantages will be apparent from the following description of presently preferred embodiments of the invention, given for the purpose of disclosure and taken in conjunction with the accompanying drawings. Not all embodiments of the invention will include all the specified advantages. For example, one embodiment may only modulate the output of a surface-emitting laser, while another only aligns an array of surface emitting lasers with an array of modulators, or other edge-receiving optical devices.